Motion detectors incorporating microwave technology are widely used in the field of security. There are two modules in this type of detector. One is a microwave detection module that radiates microwaves into a monitored area of space and receives reflected waves. If there are moving objects, then the frequency of the reflected microwave is different from the radiated microwave. By mixing the received and radiated microwaves, we can get the difference frequency (also called intermediate frequency, IF). The other module is an IF signal processing module that will amplify, digitize, and extract an IF signal.
If there is a person walking in the monitored area, then the detection module detects the frequency difference between transmitted and received microwaves and outputs the corresponding IF frequency. The IF signals are amplified, sampled, and processed by hardware circuit and algorithms of the IF processing module to determine whether there has been an intrusion. A corresponding control output can then be generated.
Typically, a prior art motion detector using microwave technology, illustrated in FIG. 1, includes the microwave detection module 110 and the IF signal processing module 120. As illustrated in the diagram of FIG. 1, the detection module 110 outputs an electrical IF signal by sensing the motion of a human body. Then, the IF signal is processed by the IF module 120. That signal is then identified by processing in a Digital Signal Processor (DSP). A corresponding control output signal can then be generated.
The circuit of FIG. 1 exhibits several problems. The IF processing module 120 adds noise to the IF signal output by the microwave detection module 110. Hence, it is very easy to miss alarms for weak signals, such as those signals generated by long range targets or having a low scattering cross section area. Additionally, if the detection module is battery-powered, then the radiated microwave power can only have limited signal strength. Therefore, detector sensitivity is a problem at times.